In conventional helical scan video recorders adapted for reproducing video signals that are modulated on a high frequency carrier and fixed on a track of an elongated magnetic tape, the tape is helically wrapped around a drum that is rotatable about its axis. A pair of circumferentially spaced magnetic pick-up heads are rotatable in a scanning path perpendicular to the axis of the drum, and serve to detect the video signals on the tape tracks as the tape is longitudinally advanced around the drum.
In order to faithfully reproduce the video signal in the playback head, it is important to assure that the video track containing the signals to be reproduced is aligned with the scanning path of the heads. Conventional techniques for assuring this essentially provide for an oscillation of each head with respect to the track. Such oscillation results in a corresponding cyclic variation in the video output from the track, which variation may then be used as an error signal for a control circuit to correct any detected misalignment of the track and head.
In copending, co-assigned application Ser. No. 346,787, filed Apr. 2, 1973, now U.S. Pat. No. 3,905,043, and entitled "Tape track-playback head alignment arrangement for a helical scan video recorder", the desired oscillation is accomplished by applying a sub-audio train of pulses to the brake of the tape advance mechanism to effect a generally triangular or sinusoidal wave modulation of the component of tape velocity parallel to the drum axis. Such cyclically varying error signal is derived from a tracking controller embodied as a pulse duration control circuit identified by the reference number 38 in the above-mentioned copending application. To accomplish this, the tracking controller is regulated by a clock pulse generator which is permanently connected in the regulating circuit so that the cyclic modulation of the error signal, and thereby a corresponding cyclic oscillation of the tape along the drum axis, is present irrespective of whether the track position is correctly located with respect to the scanning path of the playback head.
It has been found that for very high fidelity applications of the video recorder, such constant presence of the tape oscillation caused by the continuous presence of the cyclically modulated signal from the tracking controller, occasionally leads to very small but perceptible periodic disturbances of the resulting TV picture, and to a lesser extent, the accompanying sound, when the detected video program is played back.